CONTROL OF WETLAND HYDROLOGY BY BIOTIC PROCESSES

Transcription

1 HANDOUT # 2 (For 10/6 and 10/8 2004) ESP 155 Wetland Ecology, Fall 2004 Hydrology WETLAND HYDROLOGY Basic terms & concepts - most important determinant for establishing and maintenance of specific types of s and processes - despite its importance not thoroughly investigated - watershed approach Wetlands are very sensitive to changes in their regular pattern of water storage and movement Water level fluctuation => perturbation in structure CONTROL OF WETLAND HYDROLOGY BY BIOTIC PROCESSES - peat building - sediment trapping - water shading and transpiration by plants - beaver dams - gator holes HYDROLOGY - a science that deals with occurrence, distribution and movement of water HYDROLOGIC CYCLE a central concept in hydrology WATER BUDGET -the general balance between inflows, storage and outflows -mass balance equation: delta V = P n + S i + G i ET S o G o (+/-T) (not all the s have the equal proportions of all these components) GW FLOW PRECIPITATION WETLAND HYDROPERIOD - signature -- seasonal pattern of the water level of a - flood duration - flood frequency Water Depth, cm Month - primary determinants of s hydroperiod are its position on landscape (slope, flat, depression) and its source of water OVERLAND FLOW - HGM (hydrogeomorphic) functional assessment approach (Brinson & Rheinhardt 1996; Ecological Applications 6:69-76)

2 WATER BUDGET WATER BUDGET Sin P ET Sout delta V = P n + S i + G i ET S o G o (+/-T) Ideally, all components of the water budget should be measured One of the components of water budgets is frequently calculated as the residual of the budget equation. The residual contains the sum of all errors from all other components. Precipitation (P) - the main input of water to the earth's surface; liquid (rainfall and drizzle) solid (snow) for some s the only source of water GWin GWout Permanently flooded s occur where P>ET Precipitation (P) cont. Fate of precipitation: interception (I), stem flow (SF) and throughfall (TF) P = I + TF + SF Pn = P - I Pn = SF + TF Interception - in forests 5-35 %, emergent s similar Measurements of precipitation : point measurements - raingage (manual or recording) Rainfall is not regularly distributed in space and time Error from extrapolation from a single gauge station to the whole area may be as high as 75% Snow - water equivalent (equivalent water depth of melted snow) Fog 300 mm fresh snow = 25 mm rain Precipitation (P) cont. WMO - standard water gauge in each member country Coke bottles Estimating precipitation from cloud brightness (GOES = Geostationary Operational Environmental Satellite data; VISSR = Visual Infrared Spin Scan Radiometer) ground station required for data calibration Climadiagrams (Heinrich Walter) Climate diagrams are brief summaries of average climatic variabl es and their time course. The diagrams display monthly averages for temperature and precipitation over a year. 20 mm of monthly precipitation (right ordinate) equal 10 C average temperature (left ordinate). When the precipitation curve undercuts the temperature curve, the area in between them is dotted (every 2 mm) indicating dry season. When the precipitation curve supercedes the temperature curve, vertical lines are plotted for each month (with tic marks every 2 mm) indicating moist season.

3 All diagrams are designed in a uniform pattern, illustrated by the following sample: 1 Country name, station location and elevation, station name 2 The length of the observation period for temperature and precipitation respectively 3 Annual average of temperature and annual precipitation sum 4 (red) Temperature curve Overland flow SURFACE INFLOW AND OUTFLOW: Channelized flow Watershed (Catchment) outflow inflow 5 (blue) Precipitation time series 6 Indication of frost periods 7 Mean daily max. temperature of the warmest month 8 Mean daily min. temperature of the coldest month S = P * A * Rp S surface runoff P average precipitation in a watershed A area of watershed Rp hydrologic response coefficien Rp is a lump term that takes into account the surface roughness and slope of a catchment, travel time of flow, and any other factor that is going to control how fast the rainwater becomes a surface flow. This variable is estimated by checking some reference sites (residential area, forest, pasture), measuring stream discharge, rainfall intensity for a number of storms, the area of a catchment, and solving Rpfor that particular catchment. If the catchmentof interest varies in land cover, the individual parts are then averaged to provide an estimate of Rp Channelized flow inflow GROUNDWATER INFLOW AND OUTFLOW: Discharge - GW moving into Recharge - GW moving out of into aquifer Watershed (Catchment) outflow S = A * V S stremflow A cross-sectional area V average velocity Manning equation (when velocity measurement not possible) V = R 2/3 * s 0.5 /n R = hydraulic radius = crossectional area/wetted perimeter; s = slope; n = Manning coefficient concrete lined channel deep natural channel 0.03 winding stream 0.04 sluggish stream w. veget The flow of groundwater into, through and out of a described by an equation called: Darcy's law G = k. a. s, G = flow rate of groundwater (m3/day), k = hydraulic conductivity (m/d; cm/s), a = cross-sectional area, s = hydraulic gradient (delta h/delta x) - the rate of flow of water is proportional to the hydraulic conductivity or permeability and hydraulic gradient

4 Hydraulic conductivity (k) - the capacity of a soil to transmit water. It varies for different types of sometimes by several orders of magnitude. -differences in hydraulic conductivity between saturated and unsaturated soils; in saturated soils k depends on the geometry of pore spaces - k (m/d): Peat Clay Sand when calculating the GW flow, we assume that the whole area has the same k - How to measure: lab tests auger hole method: pump - depletion model fill - recharge model Hydraulic gradient - slope of water (piezometric surface, potentiometric surface) - measured in number of water table wells related to known datum (elevation) -water table wells (slotted plastic pipe) serve for identifying the elevation of the water table surface (hydraulic head) - at least two water table wells needed to quantify the direction of GW flow Evapotranspiration (ET) Water removed from soil/water by evaporation together with water passing through plants as transpiration Evaporation measured in Evaporation pans (evaporation depends on the pan design) Class A pans commonly used in National Weather Service stations Evaporation depends on: 1) solar radiation (major source of heat) 2) temperature of air and evaporating surface higher air T - more water vapor it can hold tropical areas - higher moisture 3) saturation deficit of the air -- the amount of water that can be taken by the air before it gets saturated 4) wind speed 5) evaporating surface Three approaches to estimating ET: 1) Mass transfer or water vapor flux Rate of evapotranspiration proportional to the difference between vapor pressure at the water (leaf) surface and the vapor pressure in the overlaying air. 2) Energy Balance (heat balance) 3) Empirical relationships based on both approaches (the use of meteorological constants and measurements available from meteorological stations) Penman's-Monteith; Thorntwaite; Hammer and Kadlec; etc.

5 TIDES (From Tides web site) Difference between evaporation from free-water surface and evapotranspiration ET from ranges from 0.54 to 5.3 times pan evaporation depending on plant species different stomatal conductance Lysimeter - soil block with a plant population enclosed in a container; loss of water through evapotranspiration is measured as a change in total weight An ocean TIDE refers to the cyclic rise and fall of seawater. The moon is the primary factor controlling the temporal rhythm and height of tides. At the location on the Earth closest to the moon, seawater is drawn toward the moon because of the greater strength of gravitational attraction. On the opposite side of the Earth, another tidal bulge is produced away from the moon. The second factor controlling tides on the Earth's surface is the sun's gravity. The height of the average solar tide is about 50 % the average lunar tide container w. vegetation sitting on a balance From the GSF Inst. Of Ecol. Web site Spring tide. Neap tide TYPES OF TIDES The geometric relationship of moon and sun to locations on the Earth's surface results in creation of three different types of tides. Northern Gulf of Mexico and SE Asia Atlantic coasts of the US and Europe West coast of canada and the US